The capacity to direct migration of blood-borne cells to a predetermined location (“homing”) has profound implications for a variety of physiologic and pathologic processes. Recruitment of circulating cells to a specific anatomic site is initiated by discrete adhesive interactions between cells in flow and vascular endothelium at the target tissue(s). The molecules that mediate these contacts are called “homing receptors”, and, as defined historically, these structures pilot tropism of cells in blood to the respective target tissue. At present, only three tissue-specific homing receptors are recognized: L-selectin for peripheral lymph nodes, α4β7 (LPAM-1) for intestines and gut-associated lymphoid tissue, and Cutaneous Lymphocyte Antigen (CLA) for skin (1). Apart from these tissues, it has also been recognized for several decades that circulating cells, especially hematopoietic stem cells, navigate effectively to bone marrow (2). However, extensive investigations on this process over several decades have yielded complex and sometimes conflicting results, providing no direct evidence of a homing receptor uniquely promoting marrow tropism.
From a biophysical perspective, a homing receptor functions as a molecular brake, effecting initial tethering then sustained rolling contacts of cells in blood flow onto the vascular endothelium at velocities below that of the prevailing bloodstream (Step 1) (1). Thereafter, a cascade of events ensue, typically potentiated by chemokines, resulting in activation of integrin adhesiveness (Step 2), firm adherence (Step 3) and endothelial transmigration (Step 4) (3). This “multi-step paradigm” holds that tissue-specific migration is regulated by a discrete combination of homing receptor and chemokine receptor expression on a given circulating cell, allowing for recognition of a pertinent “traffic signal” displayed by the relevant vascular adhesive ligands and chemokines expressed within target endothelium in an organ-specific manner. Following engagement of homing receptor(s) directing trafficking of cells to bone marrow, several lines of evidence indicate that one chemokine in particular, SDF-1, plays an essential role in Step 2-mediated recruitment of cells to this site (2, 4, 5).
The most efficient effectors of Step 1 rolling interactions are the selectins (E-, P- and L-selectin) and their ligands (1). As the name implies, selectins are lectins that bind to specialized carbohydrate determinants, consisting of sialofucosylations containing an α(2,3)-linked sialic acid substitution(s) and an α(1,3)-linked fucose modification(s) prototypically displayed as the tetrasaccharide sialyl Lewis X (sLex; Neu5Acα2-3Galβ1-4-[Fucα1-3]GlcNAcβ1-)) (1, 6). E- and P-selectin are expressed on vascular endothelium (P-selectin also on platelets), and L-selectin is expressed on circulating leukocytes (1). E- and P-selectin are typically inducible endothelial membrane molecules that are prominently expressed only at sites of tissue injury and inflammation. However, the microvasculature of bone marrow constitutively expresses these selectins (5, 7), and in vivo studies have indicated a role for E-selectin in recruitment of circulating cells to marrow (5, 8). Importantly, SDF-1 is constitutively expressed in high concentration within the marrow and is co-localized uniquely with E-selectin on the specialized sinusoidal endothelial beds that recruit blood-borne cells to the bone marrow (5).
Two principal ligands for E-selectin have been identified on human hematopoietic stem/progenitor cells (HSPC), PSGL-1 (9) and a specialized sialofucosylated CD44 glycoform known as Hematopoietic Cell E-/L-selectin Ligand (HCELL) (10, 11). CD44 is a rather ubiquitous cell membrane protein, but the HCELL phenotype is found exclusively on human HSPCs. In contrast to HCELL's restricted distribution, PSGL-1 is widely expressed among hematopoietic progenitors and more mature myeloid and lymphoid cells within the marrow (9). HCELL is operationally defined as CD44 that binds to E-selectin and L-selectin under shear conditions, and is identified by Western blot analysis of cell lysates as a CD44 glycoform reactive with E-selectin-Ig chimera (E-Ig) and with mAb HECA452, which recognizes a sialyl Lewis X-like epitope Like all glycoprotein selectin ligands, HCELL binding to E- and L-selectin is critically dependent on α(2,3)-sialic acid and α(1,3)-fucose modifications (10-13). On human HSPCs, HCELL displays the pertinent sialofucosylated selectin binding determinants on N-glycans (10, 12). In vitro assays of E- and L-selectin binding under hemodynamic shear stress indicate that HCELL is the most potent ligand for these molecules expressed on any human cell (10, 13). Importantly, though E-selectin is constitutively expressed on microvascular endothelium of the marrow, this molecule is prominently expressed on endothelial beds at all sites of tissue injury (e.g., sites of ischemia-reperfusion injury or trauma) or inflammation.